1. Field of the Invention
The present invention relates to a photomask. More particularly, the present invention relates to a mask blank and to a method of fabricating a photomask from the same.
2. Description of the Related Art
Photomasks are used to form fine patterns, which define a semiconductor circuit, in substrates and wafers of silicon or gallium arsenide (GaAs). The photomasks bear image patterns which are transcribed onto the substrate using an exposure process, and may be categorized as binary masks and phase shift masks (PSM). Each of these types of photomasks are manufactured from photomask blanks on which the image patterns have yet to have been formed.
A conventional binary photomask includes a transparent quartz substrate and an opaque pattern formed thereon. The opaque pattern is formed of a chromium layer and corresponds to the pattern of a semiconductor circuit to be formed on a substrate or wafer. Also, a chromium oxide layer is formed on the chromium layer as an anti-reflective layer (ARL). On the other hand, a conventional PSM includes a quartz substrate, a chromium layer disposed thereon, and phase shift material interposed between the quartz substrate and chromium layer. The phase shift material provides a phase shift effect in the PSM that allows for a higher resolution than the binary mask while using a conventional light source.
FIG. 1 is a sectional view of a photomask blank for use in manufacturing a conventional binary photomask. Referring to this figure, an opaque layer 16 and a photoresist layer 18 are sequentially formed on a light-transmissive substrate 12. The light-transmissive substrate 12 is a quartz substrate, and the opaque layer 16 is a layer of chromium.
FIGS. 2 through 4 illustrate a method of fabricating a photomask from the photomask blank shown in FIG. 1.
Referring first to FIG. 2, a photoresist pattern 18a is formed by etching the photoresist layer 18. More specifically, the photoresist layer 18 is etched by scanning an electron beam or a laser beam across the photomask blank in a raster fashion. The portions of the photoresist material exposed to the electron beam or the laser beam correspond to the circuit to be formed, and selected portions of the exposed layer of the photoresist layer 18 are removed by conventional methods.
Referring to FIG. 3, the opaque layer 16 is plasma dry etched using the photoresist pattern 18a as an etch mask to form an opaque layer pattern 16a. In general, the chromium layer and chromium oxide layer formed thereon are etched using a gaseous mixture of chlorine (Cl2) and oxygen (O2).
Referring to FIG. 4, the manufacturing of the photomask is completed by removing the photoresist pattern (18a) from the structure shown in FIG. 3.
The critical dimension (CD) of lines and spaces of the opaque layer pattern (16a) is measured. In this case, the measured CD refers to an average value of the CDs measured at several locations of the photomask. The average CD is compared to a predetermined target value. The photomask is considered to be acceptable when the average CD corresponds to or only deviates a small amount from the target value.
Also, nowadays, the design rule of the integrated circuits (ICs) of semiconductor devices is becoming smaller and smaller to meet the demand for more highly integrated semiconductor devices. A high degree of uniformity is required of the CDs of a photomask if the photomask is to be capable of being used to manufacture such highly integrated semiconductor devices. The range of the CDs (the minimum measured CD subtracted from the maximum measured CD) or the standard deviation of the measured group of CDs of a photomask is taken as a measure of the uniformity of the CDs.
In the conventional method of fabricating a photomask, Cl2 and O2 gases are used to etch the chromium layer and the chromium oxide layer, and the chromium layer and the chromium oxide layer generate CrO2xCl2x as a by-product. Note, the chromium layer and the chromium oxide layer cannot be etched using only Cl2 gas or only O2 gas. In addition, Cl2 gas and O2 gas readily react with carbon, which is a component of the photoresist. As a result, the photoresist layer is also etched in a reaction that produces CO or CCl4. Also, when the chromium layer is etched using Cl2 gas and O2 gas, the etch selectivity between the chromium layer and the photoresist layer is less than about 2:1. Therefore, a thick photoresist layer should be used so that the selected portion of the chromium layer is etched away before the photoresist layer is etched away. However, a thick photoresist layer makes it difficult to control the loading effect and to effect a global uniformity, which are important aspects in the fabricating of a photomask. In addition, it is difficult to improve on making the actual (measured) CD correspond to the design CD because a thin photoresist layer cannot be used. In other words, the process margin is limited.
A hard mask pattern, instead of a photoresist pattern, has been proposed as an etching mask to improve the CD uniformity. Methods of using such a hard mask pattern are disclosed in U.S. Pat. No. 6,472,107 and US Patent Publication No. 2003/0013023. These disclosures teach a layer of silicon as being most preferable for use as a hard mask. In addition, the disclosures mention that Ti, TiW, W, TiN, Si3N4, SiO2, or spin on glass (SOG) may also be used as the material of the hard mask.
However, Si, Ti, and W, and compounds thereof also readily react with Cl2 gas and O2 gas to produce SiCl4, TiCl4, WCl5, or WOCl4. That is, the etch selectivity between these hard mask materials and a chromium layer is very low. In addition, Si3N4, SiO2, and SOG are non-conductive. Therefore, charges are not discharged from these materials when they are scanned with an electron beam. Accordingly, distortions occur in mask patterns made from Si3N4, SiO2, or SOG. Furthermore, these materials are easily oxidized, are easily dissolved by an acid, an alkali, or a cleaning solution such as H2O or H2O2, and have poor etch selectivity in an etching process using Cl2 gas or O2 gas. By-products of the reaction between these materials and the etch gas have a boiling point that is below that of chromium. The layer exposed under the hard mask is damaged when the material of the hard mask is removed after the etching process. Additionally, these hard mask materials do not tolerate an organic stripper solution. Accordingly, the above-mentioned materials cannot serve satisfactorily as a hard mask in a process of etching a chromium and/or chromium oxide layer.